Validation of a modern second‐check dosimetry system using a novel verification phantom

Abstract Purpose To evaluate the Mobius second‐check dosimetry system by comparing it to ionization‐chamber dose measurements collected in the recently released Mobius Verification Phantom™ (MVP). For reference, a comparison of these measurements to dose calculated in the primary treatment planning system (TPS), Varian Eclipse with the AcurosXB dose algorithm, is also provided. Finally, patient dose calculated in Mobius is compared directly to Eclipse to demonstrate typical expected results during clinical use of the Mobius system. Methods Seventeen anonymized intensity‐modulated clinical treatment plans were selected for analysis. Dose was recalculated on the MVP in both Eclipse and Mobius. These calculated doses were compared to doses measured using an A1SL ionization‐chamber in the MVP. Dose was measured and analyzed at two different chamber positions for each treatment plan. Mobius calculated dose was then compared directly to Eclipse using the following metrics; target mean dose, target D95%, global 3D gamma pass rate, and target gamma pass rate. Finally, these same metrics were used to analyze the first 36 intensity modulated cases, following clinical implementation of the Mobius system. Results The average difference between Mobius and measurement was 0.3 ± 1.3%. Differences ranged from −3.3 to + 2.2%. The average difference between Eclipse and measurement was −1.2 ± 0.7%. Eclipse vs. measurement differences ranged from −3.0 to −0.1%. For the 17 anonymized pre‐clinical cases, the average target mean dose difference between Mobius and Eclipse was 1.0 ± 1.1%. Average target D95% difference was ‐0.9 ± 2.0%. Average global gamma pass rate, using a criteria of 3%, 2 mm, was 94.4 ± 3.3%, and average gamma pass rate for the target volume only was 80.2 ± 12.3%. Results of the first 36 intensity‐modulated cases, post‐clinical implementation of Mobius, were similar to those seen for the 17 pre‐clinical test cases. Conclusion Mobius correctly calculated dose for each tested intensity modulated treatment plan, agreeing with measurement to within 3.5% for all cases analyzed. The dose calculation accuracy and independence of the Mobius system is sufficient to provide a rigorous second‐check of a modern TPS.

MobiusFX allows for quality assurance of the treatment plan by offering a "delivered dose" calculation, generated using the Mobius3D model and delivery log files produced by the treatment machine. In addition, the MobiusFX module provides automatic "de- In order for a second-check dosimetry system to provide useful information to the user, the user must have confidence in the ability of the second-check system to accurately calculate dose. While second-check systems are not expected to approach the dosimetric accuracy of the primary TPS, their accuracy must be such that the number of "false positives" produced does not obscure any true problems with the primary TPS. Ideally, a treatment plan failing a second-check dosimetry calculation should be an unusual occurrence, prompting a thorough investigation. If the second-check system lacks the ability to accurately calculate dose for complex modern treatment plans, failures will be routine and indeterminate, For this study, treatment log files were used solely to facilitate calculation of dose on the MVP within the MobiusFX module. Mobius utilizes the delivery information, reported in the log-file, when calculating dose within the MobiusFX module. There is an ongoing discussion within the medical physics community regarding the validity of log-file analysis for patient-specific quality assurance (QA) of modulated treatment beams. [10][11][12] Publications examining the reliability and effectiveness of this QA method are available for review. [13][14][15][16][17][18][19][20] 2 | METHODS

2.A | Mobius commissioning
The Mobius beam model is created primarily using accelerator-speci- In Eclipse, the DLG is a multi-leaf collimator (MLC) positional offset designed to account for additional beam transmission through MLCs with rounded leaf tips. 21 In Mobius, the DLG correction factor is an additional factor added to an internal DLG already present in the Mobius system. The internal DLG is not visible to the user.
Mobius Medical Systems recommends using ionization chamberbased dose measurements to determine the optimal DLG correction factor. Prior to beginning this study, the DLG correction factor was optimized for each Mobius beam model, following the vendor's recommended procedure. The results reported here were calculated using optimum DLG correction values of 1.1 and À0.7 mm for 6 and 10 MV, respectively. Readers attempting to replicate our results should first determine the optimal DLG correction value for their specific treatment machine.  Table 1.

2.B | Plan selection
For the post-implementation clinical plan comparison, the first 36 treatment plans evaluated using Mobius were chosen for inclusion in this study. The only requirement for inclusion was intensity modulation; this included IMRT as well as VMAT. Of these 36 clinical cases, 33 used a beam energy of 6 MV, one used a beam energy of 10 MV, and two used mixed energy (6 and 10 MV).

2.C | Pre-clinical validation: determination of phantom dose in eclipse
For each of the 17 selected treatment plans, a verification plan was generated in the Eclipse treatment planning system. Verification plans were created by copying the beams or arcs onto a CT-dataset of the MVP TM while maintaining the original MLC segments and beam monitor units. As described previously, the MVP is a Virtual Water TM phantom with seven available positions for an ionization chamber, labeled A through G, as well as a space for film insertion if needed. Our MVP has been drilled for use with an ADCL-calibrated Standard Imaging  analyzed plans can be found in Table 3. For reported percent differences, a positive percent difference indicates that Mobius calculated a higher number for a given statistic. For both the 3D global and target gamma pass rates, gamma criteria of 3%/2 mm were used.
Mobius tended to report a higher target mean dose than AcurosXB, with an average target mean dose percent difference of 1.0 AE 1.1%.
Target mean dose differences ranged from À1.2 to 2.6%. Mobius tended to report a lower target D95% than AcurosXB, with an average target D95% difference of À0.9 AE 2.0%. Target  kernels. 1 AcurosXB calculates dose to medium, requiring accurate assignment of materials within the body to incorporate the atomic properties of each material into the dose calculation. 26 Because of the fundamentally different method each algorithm uses to account for heterogeneities, we expect to see dose differences in heterogeneous areas and at boundaries between differing materials. Figure 4, taken from plan 16, demonstrates each of these scenarios. Note the failing gamma points at each beam entrance due to differences in the build-up model. In addition, multiple areas within the lung and bone also show failing gamma, likely due to differences in the way each algorithm handles inhomogeneities and re-build-up of dose.
Further analysis is necessary to quantitate differences between the AcurosXB and Mobius dose algorithms due to patient anatomy.

| CONCLUSION
The Mobius second check system was validated for a variety of intensity modulated clinical treatment plans, using the recently released MVP and an ADCL-calibrated ionization chamber. We found that Mobius performed well compared to the AcurosXB dose calculation algorithm, implemented in our clinical treatment planning system, Eclipse. Mobius differences from measured dose were reasonable, especially in the context of a second check system utilizing a non-customized beam model. We feel that intensity modulated plans can be safely verified using the Mobius system, and that extreme differences between Mobius and Eclipse would rightly prompt a thorough examination of the plan in question. The differences that are seen between Mobius and Eclipse are somewhat predictable, and are likely due to differences between the customized F I G . 4. Mobius vs. Eclipse AcurosXB gamma map for selected treatment plan. Mobius vs. Eclipse AcurosXB gamma map (3%, 2 mm) is shown for treatment plan 16. Yellow and blue areas represent high and low gamma failures, respectively. Note the typical areas of gamma failure at each beam entrance due to differences in the build-up model, and heterogeneous areas in the lung and bone.

CONFLICT OF INTEREST
The authors have no conflicts of interest to report.